Type II Supernova Spectral Diversity. I. Observations, Sample Characterization, and Spectral Line Evolution
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AbstractWe present 888 visual-wavelength spectra of 122 nearby type II supernovae (SNe II) obtained between 1986 and 2009, and ranging between 3 and 363 days post-explosion. In this first paper, we outline our observations and data reduction techniques, together with a characterization based on the spectral diversity of SNeII. A statistical analysis of the spectral matching technique is discussed as an alternative to nondetection constraints for estimating SN explosion epochs. The time evolution of spectral lines is presented and analyzed in terms of how this differs for SNe of different photometric, spectral, and environmental properties: velocities, pseudo-equivalent widths, decline rates, magnitudes, time durations, and environment metallicity. Our sample displays a large range in ejecta expansion velocities, from 9600 to 1500 km s1 at 50 days post-explosion with a median m{H}}}_{alpha }$?> value of 7300 km s1. This is most likely explained through differing explosion energies. Significant diversity is also observed in the absolute strength of spectral lines, characterized through their pseudo-equivalent widths. This implies significant diversity in both temperature evolution (linked to progenitor radius) and progenitor metallicity between different SNeII. Around 60% of our sample shows an extra absorption component on the blue side of the m{H}}}_{alpha }$?> P-Cygni profile (Cachito feature) between 7 and 120 days since explosion. Studying the nature of Cachito, we conclude that these features at early times (before 35 days) are associated with Si ii, while past the middle of the plateau phase they are related to high velocity (HV) features of hydrogen lines.
